This invention pertains to the catalytic reduction of nitrogen oxide and dioxide (collectively NOx) in the exhaust gas from a lean burn hydrocarbon-fueled engine or under similar oxygen and water containing atmosphere. More particularly, this invention pertains to the use of a hydrothermally stable zeolite in cation-exchanged form for such purpose.
In order to further improve the fuel efficiency of hydrocarbon fuel engines there is interest in operating the engine in a fuel-lean combustion mode. For gasoline engines this means introducing an air/fuel mixture at a ratio of about seventeen to twenty three parts by weight of air per part of gasoline. For diesel engines the air to fuel mass ratio is even higher. The purpose of fuel lean operation is to obtain more complete combustion of the fuel.
Attractive as the lean-burn engines have become lately for their superior fuel efficiency, there remains a major technical barrier to the automotive application of lean-burn engine technology. It is associated with NOx emission in the engine exhaust. The exhaust gas from a lean-burn gasoline engine is typically at a temperature of 300xc2x0 to 600xc2x0 C. during warmed up engine operation. And the exhaust contains water, small amounts of carbon monoxide and unburned hydrocarbons (e.g., ethylene), nitrogen, and nitrogen oxides (NO and NO2). The challenge is to promote the reduction of NOx in this chemically oxidizing environment.
The traditional three-way catalysts while active for NOx reduction under stoichiometric exhaust conditions, are not effective in reducing NOx under highly oxidizing conditions prevailing in the lean-burn engine exhaust. Lean-NOx reduction technologies currently available are not sufficiently effective to meet future stringent emission standards either. This has prompted intensive and extensive RandD activities around the world for improved lean-NOx reduction technology.
Among a few different approaches for lean-NOx reduction, the selective catalytic reduction of NOx using unburned hydrocarbons (HC-SCR) as reductants has been attracting the most attention. There are quite a few known lean-NOx reduction catalysts for the HC-SCR process. Among those reported in the literature, Cu/ZSM-5 zeolite is probably the most studied catalyst for high temperature applications, whereas Pt/ZSM-5 is for low temperature applications. In most lean-NOx catalysts, zeolites are used as catalyst support on which the active metals are ion exchanged. Among many different zeolites, the ZSM-5 zeolites with high silica content have been preferentially used for lean-NOx catalysts. Unfortunately, however, all those catalysts suffer from the combination of the narrow effective operating temperature window and insufficient catalytic activity and hydrothermal stability.
All zeolite-based catalysts, Cu/ZSM-5 in particular, have major problems due both to hydrothermal degradation and negative sensitivity towards water vapor and SO2. In general, the permanent loss of activity has been attributed by investigators to (a) degradation of the support, (b) irreversible loss of Cu2+ from the zeolite framework or (c) combination of the above. The Cu1+ is known to be the active catalytic site for both NO decomposition and NO reduction with hydrocarbon. The inter-conversion between Cu1+ and Cu2+ depends on the reaction conditions including temperature and the types of reductant. Hydrothermal de-alumination of the zeolite framework has been a major issue in the deactivation of the catalyst. It appears that deactivation is mainly caused by migration of Cu2+ ions to locations inside ZSM-5 where their reduction to Cu1+ is more difficult. The above mentioned studies clearly reveal that Cu/ZSM-5 deactivates substantially even under relatively mild conditions and indicate that a dramatic increase in hydrothermal stability is required for the catalysts if they are to be used in the automotive application. Thus, the search continues for better lean-NOx catalysts, which requires both more stable supports and more active catalytic chemical ingredients.
Accordingly, it is an object of this invention to provide a stable and effective catalyst for reduction of NOx in a lean burn exhaust such as from a hydrocarbon-fueled automotive vehicle engine.
This invention utilizes certain metal ion exchanged SUZ-4 zeolites to catalyze the reduction of nitrogen oxides in a high temperature gas mixture also containing nitrogen, water and small amounts of carbon monoxide and unburned hydrocarbons. U.S. Pat. No. 5,118,483 to Barri, entitled Crystalline (Metallo) Silicates and Germanates-SUZ-4 describes the synthesis of a family of materials having a porous crystalline structure. Some of the synthetic aluminosilicate members (zeolites) of this family are useful in the practice of this invention.
The suitable SUZ-4 zeolite starting materials are crystalline aluminosilicates. They have an empirical formula in their dehydrated form of M2O:Al2O3:ySiO2. The cation M in the SUZ-4 starting material is preferably an alkali metal cation selected from Li+, Na+, K+ or Cs+ and y has a value such that the ratio of Si to Al is in the range of about 5.1 to 6. In general, zeolites have complex crystalline structures with pores and/or channels of specific and uniform dimensions. These structures are largely dependent upon the synthesis of the zeolite. And retention of these structures during NOx reduction operating conditions is necessary for stability of a catalyst. Thus the ""483 patent states the process for the preparation of SUZ-4 materials requires the presence of tetraethylammonium hydroxide or halide or its precursor or reaction product as a template, and that other nitrogenous materials may be present in the reaction mixture. These SUZ-4 synthesis components are, of course, in addition to suitable quantities of alumina and silica precursors and the M cation(s).
SUZ-4 zeolites have been found to be surprisingly stable at temperatures up to 800xc2x0 C. in a flowing nitrogen atmosphere containing five percent oxygen and 2.5 percent water. Since all other known zeolites, including ZSM-5, have experienced degradation under these extreme hydrothermal conditions it was decided to further evaluate certain cation exchanged SUZ-4 zeolites as catalysts for the reduction of nitrogen oxides under like conditions representative of the exhaust of a lean burn automotive engine.
Potassium SUZ-4 zeolite was synthesized. Copper (II), silver (I), iron (III) and cobalt (II) ion exchanged SUZ-4 zeolites were prepared, each by the aqueous ion exchange method. In each case a portion of the potassium ion content in the SUZ-4 zeolite was replaced with one of these cations. Each of these cation exchanged materials was tested for catalytic activity in synthetic C2H4xe2x80x94NOxe2x80x94O2 feedstreams (helium background) using a packed bed reactor under steady state conditions over a wide temperature range from 200 to 650xc2x0 C.
Each of these cation exchanged SUZ-4 zeolites was effective in reducing nitrogen oxides in the oxygen containing gas that simulated a lean burn exhaust. The copper exchanged SUZ-4 material was especially effective in reducing nitrogen oxides over a broad range of operating temperatures even in the presence of water and/or SO2. The optimum range of copper (II) ion exchange was from 29 to 42% of the ion exchange capacity of the potassium SUZ-4 zeolite. Potassium ions remained as the balance, 58 to 71%, of the cation content.
In addition to their catalytic activity in reducing nitrogen oxides in a hydrocarbon containing but oxygen rich environment, these cation substituted SUZ-4 zeolites retained their effectiveness after hydrothermal ageing.
Other objects and advantages of the invention will become apparent from a description of preferred embodiments which follows.